1. Field of the Invention
The present invention relates to a method for balancing a shaft-and-wheel assembly of a turbocharger. More particularly, the present invention relates to a method of removing balance stock from a mixed flow turbine wheel having a partial back-wall.
2. Description of Related Art
A turbocharger is a type of forced induction system used with internal combustion engines. Turbochargers deliver compressed air to an engine intake, allowing more fuel to be combusted, thus boosting an engine's horsepower without significantly increasing engine weight. Thus, turbochargers permit the use of smaller engines that develop the same amount of horsepower as larger, normally aspirated engines. Using a smaller engine in a vehicle has the desired effect of decreasing the mass of the vehicle, increasing performance, and enhancing fuel economy. Moreover, the use of turbochargers permits more complete combustion of the fuel delivered to the engine, which contributes to the highly desirable goal of a cleaner environment.
Turbochargers typically include a turbine housing connected to the engine's exhaust manifold, a compressor housing connected to the engine's intake manifold, and a center bearing housing coupling the turbine and compressor housings together. A turbine wheel in the turbine housing is rotatably driven by an inflow of exhaust gas supplied from the exhaust manifold. A shaft rotatably supported in the center bearing housing connects the turbine wheel to a compressor impeller in the compressor housing so that rotation of the turbine wheel causes rotation of the compressor impeller. The shaft connecting the turbine wheel and the compressor impeller defines an axis of rotation. As the compressor impeller rotates, it increases the air mass flow rate, airflow density and air pressure delivered to the engine's cylinders via the engine's intake manifold.
There are three basic types of turbine wheels. The radial turbine wheel has the fluid flowing around the edge of the turbine wheel. An example of such a wheel is a water wheel. An axial turbine has the fluid flowing through the turbine blades. A windmill is an example of an axial turbine. The mixed flow turbine wheel combines the designs of both the axial flow and radial flow turbines. The present invention relates to mixed flow turbine wheels.
The turbine wheel operates in a high temperature environment and the turbine wheel may reach temperatures as high as 1922° F. (1050° C.). This elevated temperature can have an effect on material properties turbine wheel and make it less able to withstand stress. In addition, the turbine wheel of a turbocharger rotates very fast. The rotation speed of a turbine wheel is size dependent and smaller turbine wheels can rotate faster than larger wheels. A small turbocharger turbine wheel used in conjunction with an internal combustion engine may reach rotational velocities as high as 350,000 RPM. The rapid rotation of the turbine wheel creates large centrifugal forces or centrifugal stress on the wheel. The combination of the high temperature operating environment and the high rotation speed make balancing the turbine wheel extremely important. In addition the turbine wheel is heavy and made from expensive materials. Any balance problems can lead to early failure and the loss of the expensive turbine wheel and shaft.
The turbine wheel is one of the most expensive components of the turbocharger because it is typically cast from a nickel based superalloy with over seventy percent (70%) by weight in nickel. This equates to approximately five percent (5%) of the weight of the entire turbocharger. Thus, it is desirable for the turbine wheel to have a long lifecycle. Lack of balance in the turbine wheel can cause vibrations which may be transmitted the rest of the turbine and thus the rotational balance of the turbine wheel is critical for both performance and lifecycle of both the turbine wheel and the entire turbocharger.
However, the rotational balance of the turbine wheel is unknown until it is part of a finished shaft-and-wheel assembly, and unfortunately, the balancing step is generally the last operation in the manufacture of the shaft-and-wheel assembly. For one example, a turbine wheel casting may be held in a chuck to drill a center hole in a nose on a front side of the turbine wheel casting. The shaft is then welded to a weld boss on a back side of the turbine wheel casting. After heat treating the weld, the shaft-and-wheel assembly is machined, including finish machining a plurality of turbine blades on the turbine wheel itself. A distal end of the shaft is threaded and then the shaft-and-wheel assembly is balanced. If the shaft-and-wheel assembly must be scrapped, at this point, due to balance problems, there is a large non-recoverable cost.
The turbine wheel may be balanced by removing metal from the back wall of the turbine wheel. A fullback turbine wheel is a wheel having a back-wall having a hubline that extends all the way to an inlet tip of the turbine blade, thereby defining an outer diameter. In a fullback turbine wheel design, there is a great deal of material which can be removed from the back in order to balance the turbine wheel. The mixed flow turbine wheel design leaves little metal which can be removed from the back wall of the turbine wheel. Removing material farther from the axis of rotation has greater impact on the inertia of a turbine wheel than removing material closer to the axis. Accordingly, scallop cuts provide good reduction of inertia in fullback turbine wheels suitable for use as radial turbine wheels. Stock removal from the smaller back of mixed flow wheels has less effect on the moment of inertia.
Scallop cuts in the back wall of the turbine wheel, between the turbine wheel blades have been used to reduce the inertia of the turbine wheel. Examples of such wheels are disclosed in U.S. Pat. No. 7,771,170.
U.S. Pat. No. 6,471,474 relates to a rotor assembly for a gas turbine engine operating with reduced circumferential rim stress. The rotor assembly includes a rotor including a plurality of rotor blades extending radially outward from an annular rim. A root fillet extends circumferentially around each blade between the blades and rim. The rim includes an outer surface including a plurality of concave indentations extending between adjacent rotor blades and forming a compound radius. Each indentation extends from a leading edge of the rotor blades towards a trailing edge of the rotor blades.
U.S. Pat. No. 6,511,294 relates to a gas turbine engine rotor assembly including a rotor having a radially outer rim with an outer surface shaped to reduce circumferential rim stress concentration between each blade and the rim. Additionally, the shape of the outer surface directs air flow away from an interface between a blade and the rim to reduce aerodynamic performance losses between the rim and blades. In an exemplary embodiment, the outer surface of the rim has a concave shape between adjacent blades with apexes located at interfaces between the blades and the rim.
U.S. Pat. No. 6,524,070 relates to a rotor assembly for a gas turbine engine operating with reduced circumferential rim stress. The rotor assembly includes a rotor including a plurality of rotor blades and a radially outer platform. The rotor blades extend radially outward from the platform. A root fillet extends circumferentially around each blade between the blades and platforms. The platforms include an outer surface including a plurality of indentations extending between adjacent rotor blades. Each indentation extends from a leading edge of the platform to a trailing edge of the platform with a depth that tapers to an approximate zero depth at the trailing edge.
U.S. Pat. No. 6,942,460 relates to a radial turbine impeller, comprising a circular main disk provided with a plurality of blades, each having a negative pressure surface and a positive pressure surface; scallops being formed by cutting off the main disk between the negative pressure surface of the one blade and the positive pressure surface of the other blade adjacent to the one blade, respectively; wherein a minimum radius portion of the scallop having a minimum distance between a center of the circular main disk and the edge of the scallop is positioned closer to the positive pressure surface so that the scallop is asymmetric between the negative pressure surface of the one blade and the positive pressure surface of the other blade adjacent thereto.
U.S. Pat. No. 7,465,155 relates to a turbomachine blade row having a hub that includes a non-axisymmetric end wall modified by a transformation function. The blade row further includes a circumferential row of a plurality of airfoil members radially extending from the non-axisymmetric end wall of the hub and forming a plurality of sectoral passages therebetween. A radius of the non-axisymmetric end wall is determined by a transformation function including a plurality of geometric parameters defined by a user based on flow conditions. The plurality of geometric parameters provide for modification of the end wall in both an axial and a tangential direction to include a plurality of concave profiled regions and convex profiled regions.
U.S. Pat. No. 7,771,170 relates to a turbine wheel in which the hub/blade junction of each rotor blade is placed with respect to the scalloping surface (F1+F2) such that this surface is supported as symmetrically as possible by the rotor blade. The turbine wheel with three-dimensionally curved rotor blades has scalloping in the area of the hub rear wall, and in consequence is subject to reduced stresses caused by scalloping deformation.